A hypothesis is presented to explain cell recognition, selective cell adhesion and cell sorting out in embryonic cells, the invasiveness of normal tissue by tumor cells, and the general problem of metastasis of cancer cells. The hypothesis states that cells sort out by non-random movement leading to a segregation of cells according to species and/or cell type. This sorting out is accompanied by the formation of microvilli in specific response to neighboring cells and the production of a specific extracellular material to bind similar cells together. Studies are proposed to test the hypothesis among which are: experiments to further demonstrate the non-random movement of cells during sorting out, the role of the extracellular material and the cell surface in selective cell adhesion, the use of hybrid embryos to detect paternal gene activity in cell sorting out, chemotaxis and its role in sorting out, and the effect of cytochalasin B on selective cell adhesion. Embryonic sea urchin cells will be used as the primary experimental material. These studies are fundamental to an understanding of normal morphogenesis but also to abnormal events such as malignancy.